| OCR Text |
Show 1893.] MR. M. F. WOODWARD ON MAMMALIAN DENTITION. 461 the premaxillo-maxillary suture (incisor fissure). The condition of the premolars is very interesting. In the younger of the two specimens examined, the rudiment of the successional tooth is developed very much as in Petrogale, that is in the interspace between pm? and pm4, but it is much closer to the former, with which it is connected, and has no connection whatever at this stage with the latter; the difference may be, howrever, due to the fact that this is a slightly older stage than that described for Petrogale. In the second stage this rudiment of the successional premolar is much more differentiated, and owing to the development of pm3, which is here very large, it eomes to be under the posterior end and on the inner side of this tooth, wdth which we have seen it is undoubtedly connected. One thing we may say for certain, that in M. brachyurus the solitary successional tooth is not developed in connection with pm4, but either represents a tooth in between pm4 and pm3 or else is the successor to the penultimate premolar (pm3). The relation of the premolars in the lower jaw is the same as that in the upper. MACROPUS EUGENII. Two specimens, 125 m m . long. In this species there are present in the upper jaw 5 pairs of incisors, corresponding with i\ i2, i4, i5, and i6 of Petrogale penicillata and, as in that form, i\ i4, and ie are the functional adult teeth, while i2 and iB are vestigial structures. N o trace of that very rudimentary tooth i3 could be found. Of the rudimentary teeth t* is very small, this being in all probability due to the fact that the first functional incisor here early attains a large size, and has consequently further dwarfed the disappearing i2. O n the other hand, i° is a very prominent and well-developed tooth (fig. 5), and in conformity with this we find that i6 is at present very slightly developed and quite uncalcified. A transverse section across the jaw and passing through i5 and ia is shown in fig. 5, and it will be seen there is a well-developed downgrowth from the inner side of the enamel-organ of ie, showing that this latter tooth, like il and i4, belongs to the 1st dentition, the downgrowth (ie') representing the rudimentary permanent tooth. The small tooth i5 is therefore quite an independent tooth, and probably represents the 5th incisor of the first dentition. Of the first lower incisor (ix) no trace was observed in this species ; the second and functional incisor was large and calcified, and exhibited a large and definite rudiment of its successional tooth in the form of a thick cord-like downgrowth from the inner side of its enamel-organ, this rudiment being more definite here than in any other form I have examined. The upper canine is a small tooth situated in the maxilla and close to the last incisor; it shows no indication of a successional tooth, nor in any form which I have examined can one say |
